Impedance transformation in a duplexer using a transmission line

Abstract

A duplexer having a filter and a transmission line that transforms an impedance of the filter to an impedance of an antenna.

1. A duplexer comprising a filter and a transmission line that transforms an impedance of the filter to an impedance of an antenna, wherein the transmission line has an impedance that is a geometric mean of the impedances of the filter and the antenna, wherein the geometric mean of the impedance is given by:
     Z   L =( Z   RX   *Z   A ) 1/2    
 where Z L  is the impedance of the transmission line, Z RX  is the impedance of the filter, and Z A  is the impedance of the antenna. 
 
     
     
       2. The duplexer of  claim 1 , wherein the transmission line is a quarter wave transmission line at a frequency of a transmit filter of the duplexer. 
     
     
       3. The duplexer of  claim 1 , wherein the filter is a receive filter. 
     
     
       4. The duplexer of  claim 1 , wherein a length of the transmission line is selected to provide a 90 degree phase shift in a transmit band of a transmit filter of the duplexer. 
     
     
       5. The duplexer of  claim 1 , wherein the impedance transformation provided by the transmission line enables the use of higher impedance structures on a receive side of the duplexer. 
     
     
       6. A method for impedance matching in a duplexer, the method comprising: transforming an impedance of a filter in the duplexer to an impedance of an antenna, wherein the transforming comprises:
 selecting an impedance of a transmission line to have a geometric mean of the impedance of the filter and the impedance of the antenna, wherein the geometric mean is given by:
     Z   L =( Z   RX   *Z   A ) 1/2    
 
 where Z L  is the impedance of the transmission line, Z RX  is the impedance of the filter, and Z A  is the impedance of the antenna. 
 
     
     
       7. The method of  claim 6 , wherein transforming comprises transforming using a quarter wave transmission line at a frequency of a transmit filter of the duplexer. 
     
     
       8. The method of  claim 6 , wherein transforming comprises transforming an impedance of a receive filter in the duplexer to the impedance of the antenna. 
     
     
       9. The method of  claim 6 , further comprising selecting a length of the transmission line to provide a 90 degree phase shift in a transmit band of a transmit filter of the duplexer. 
     
     
       10. A cellular communication station, comprising:
 an antenna; 
 a filter; and 
 a duplexer having a transmission line that transforms an impedance of the filter to an impedance of the antenna, wherein the transmission line has an impedance that is a geometric mean of the impedance of the filter and the impedance of the antenna, wherein the geometric mean is given by:
     Z   L =( Z   RX   *Z   A ) 1/2    
 
 where Z L  is the impedance of the transmission line, Z RX  is the impedance of the filter, and Z A  is the impedance of the antenna. 
 
     
     
       11. The cellular communication station of  claim 10 , wherein the transmission line is a quarter wave transmission line at a frequency of a transmit filter of the duplexer. 
     
     
       12. The cellular communication station of  claim 10 , wherein the filter is a receive filter. 
     
     
       13. The cellular communication station of  claim 10 , wherein a length of the transmission line is selected to provide a 90 degree phase shift in a transmit band of a transmit filter of the duplexer. 
     
     
       14. The cellular communication station of  claim 10 , further comprising a structure on a receive side of the duplexer having a higher impedance than the impedance of the antenna.